AMPHIBIANS

AMPHIBIANS

Station 1. Major groups of amphibians.

Phylum: Chordata
Subphylum:
Vertebrata
Superclass:
Gnathostomata
Class:
Lissamphibia
Order:
Gymnophiona (no limbs) Also known as Caecilia or
Apoda (meaning literately: no limbs)
Order:
Caudata
(tails) Also known as Urodela (salamanders)




Order: Anura (no tails) frogs and toads




The modern amphibians (Amphibia) are a monophyletic group that contains three very
distinct major lineages. They are united by the following characteristics:
- shortened ribs
- manus (hands…in those that have them) with four digits (vs. 5 in other tetrapods)
- poison glands in skin
- significant gas exchange through the skin
- glandular skin that lacks hair, scales, or feathers

The Order Gymnophiona (caecilians; ca. 175 species) are elongate, legless, animals.
They are usually terrestrial burrowers, but some are aquatic. They are characterized by:
-
the absence of girdles and limbs
- short
tails
-
segmented and greatly elongated bodies
-
reduced eyes that are covered by skin
-
protrusible sensory tentacle between the eye and nostril

The Order Caudata (salamanders; ca. 400 species) are elongate, long-tailed, and
mostly terrestrial and secretive. They usually have four functional limbs. Additionally,
salamanders are characterized by:
-
having a head, trunk and tail region that are distinct with the suggestion of a neck
-
if the limbs are present, they are sub-equal
-
fertilization is internal
-
metamorphosis is not pronounced
-
larvae if present, resemble adults and have teeth in both jaws

The Order Anura (frogs and toads; ca. 3700 species) are tailless and stout. They are
characterized by:
-
a toothless dentary (lower jaw)
-
loss of tail in adults
-
urostyle formed from the caudal vertebrae
-
elongate and anteriorly directed ilium
-
hindlimbs are longer than forelimbs (limbs sub-equal)
-
radius is fused with the ulna and tibia is fused with the fibula
-
fertilization is usually external
-
larvae differ from adults (tadpoles) and are usually aquatic
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-
no true teeth are present in the larval stage
-
metamorphosis is pronounced

You should be able to classify any amphibian into one of these three Orders!

Station 2. Anatomy.
There are two goals to this station: 1, to become familiar with the external anatomy of
amphibians and 2, to point out some evolutionary trends in the skeletal system. Goal 1:
Use Figures 2.1 and 2.2 to identify all of the external structures on the frog/toad and
salamander at this station.




Figure 2.1 External anatomy of a frog and a toad (from www.kentuckyawake.org)

Figure 2.2 External anatomy of a salamander (from www.kentuckyawake.org)
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Goal 2: Several aspects of the skeleton changed during the transition to tetrapods (4-
limbed vertebrates). Notice the pelvic and pectoral girdles. In fishes, these regions
acted as stable bases for the motion of the fins and fin elements. In tetrapods they are
used to assist in transferring the weight of the body onto the limbs. The head is also free
to move around in tetrapods versus fishes because the pectoral girdle is no longer
joined to the skull. Use Figure 2.4 to identify the urostyle (a unique element formed by
the fusion of the sacral and caudal vertebrae), ilium and fused bones (radius+ulna,
tibia+fibula) on the frog skeleton. Compare the frog skeleton, to that of a salamander
(Figure 2.3). Why might these changes have occurred (think about the lives of frogs
and toads)?

Figure 2.3 Dorsal
view of a
salamander
skeleton (from, Five
Kingdoms, 2nd ed.,
by Lynn Margulis
and Karlene V.
Schwartz, W.H.
Freeman and
Company: New
York, 1988.).


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Figure 2.4 Dorsal view of a frog skeleton (from Gergus & Schuett 1997, Labs for Vertebrate Zoology:
an Evolutionary Approach, Biological Sciences Press).

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Station 3. North American families of salamanders (Caudata).
At this station are representatives of most of the families of caudates (salamanders) in
North America. You should be able to identify any specimen to family.

Ambystomatidae (e. g., tiger salamanders). Medium to large terrestrial salamanders
(up to 340 mm in Ambystoma tigrinum ), occurring only in North America from southern
Canada south to Mexico City. This family includes metamorphosing as well as
facultative and obligate paedomorphic (maintaining juvenile traits as adults) species,
such as the famous Axolotl (Ambystoma mexicanum). Characters: costal grooves on
body; 4 toes on front feet; 5 toes on hind feet; vomerine teeth may be in a transverse
orientation in the mouth.

Amphiumidae (amphiumas). Amphiumas are large, aquatic, eel-like salamanders (up
to 1 m), with much reduced limbs, native to eastern North America.
Characters: limbs and toes reduced (3, 2, or 1 toe per foot – species-specific character);
tail is laterally compressed and makes up one third of the total body length.

Plethodontidae (Lungless salamanders)
Generalized in body form with four fingers and five toes (except when reduced to four
toes in miniaturized species).The largest family of extant salamanders with over 350
species. All members of this assemblage lack lungs and possess naso-labial grooves
which aid in chemoreception. Characters: nasolabial groove present (furrow from nostril
to edge of upper lip; might need a microscope or magnifying glass); elongate, up to 60
vertebrae; lungless; direct development; costal grooves visible.

Proteidae (e. g., waterdogs and mudpuppies; Necturus). Aquatic paedomorphic
salamanders with filamentous large red, external gills and laterally compressed tail fins.
They are distributed in Europe (Proteus, up to 200 mm) and North America (Necturus,
up to 400 mm). Characters: two pairs of larval gill slits; external gills present; reduction
in number of toes.

Salamandridae (true salamanders, e. g., newts)
Small to medium sized (Salamandra and Pleurodeles reach more than 200 mm)
terrestrial or aquatic salamanders. Most species have toxic skin secretions. Many
salamandrids develop dorsal body and tail fins when they return to an aquatic stage,
while rough skin is present in the adult newts. Lungs are present in juveniles and adults.
Characters: vomerine teeth extend posteriorly on lateral edges of vomers; frontal-
squamosal arch present; indistinct costal grooves; rough skin whereas other
salamanders have smooth skin (except in breeding males).

Sirenidae (sirens) Elongate small to large (750 mm) aquatic salamanders with external
gills. The pelvic girdle and hind limbs are absent in all species but small forelimbs are
present. They are distributed in the southern United States and northern Mexico and
usually are considered to be the sister group to all other living salamanders. Members
of this family are paedamorphic, retaining larval features as adults such as gills, a lateral
line system, and suction feeding (although they also possess moving jaws). Characters:
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maxillae reduced to tiny-free elements (notice how narrow the top of the mouth tends to
be); no pelvic girdle or hind legs; no premaxillary or maxillary teeth.

Station 4. North American families of frogs and toads (Anura).

Ascaphidae (tailed frogs)
This group is composed of two species that inhabit cold, fast flowing streams from
British Columbia, south to Mendocino County, CA and east to the Rocky Mountains of
Idaho and Montana. The most striking feature of this group is their external copulatory
organ (an everted extension of the cloaca) that is used during internal fertilization that
takes place under water in fast flowing streams. They are small, brown or gray, have
reduced lungs, and vertical pupils. Characters: maxilla and premaxillae contain teeth;
pupil vertically elliptical; internal fertilization; tail-like copulatory organ in males.

Bufonidae (True toads)
This group is large and very complicated. Bufonidae is composed of 33 genera with a
cosmopolitan distribution, except for Australia where the only Bufonid is the introduced
Cane Toad (Bufo marinus). This family includes the stereotypical “toad” with shortened
forelimbs and hind limbs used for walking or hopping, dry warty skin, and parotoid
glands behind eyes. Development in this group is varied; there are species that lay eggs
in water and produce aquatic larvae, terrestrial direct developers, and viviparous
species. Characters shared within the family are: teeth absent from upper and lower
jaws; inguinal fat bodies present; skull highly ossified (usually skin is ossified to the
skull); “warts”; horizontal pupil; distinct parotid glands; short-legged.

Hylidae (e. g., treefrogs, chorus frogs)
A wide-spread family especially represented in the new world, they range from small to
large in size and usually have distinct adhesive toe discs that contain a cartilage that
offsets the terminal phalanx, which may aid in climbing.
Characters: long-legged; lack parotid glands; smooth skin (no warts); often have toe
pads (arboreal); if lacking toe pads, are semi-aquatic with partially webbed hindfeet (e.
g., Pseudacris, Acris).

Microhylidae (e. g., narrowmouth toads)
A complicated group with 67 genera and over 300 species. Small to medium sized (up
to 100mm) terrestrial or arboreal frogs distributed throughout the Americas, Asia, Africa
and Madagascar. Species are similar in body plans through the presence of stout hind
legs, short snout, and globose bodies (globe-like). They have a generalized teardrop
shape. Characters: small, plump, short legs, pointed head, and a fold of skin across the
back of the head.

Pelobatidae (spadefoot toads)
Members of this family are collectively known as the “spade foot toads” due to the
presence of a keratinized “spade” on the hind limbs which aid in burrowing. Pelobatids
inhabit arid habitats and are known as “explosive” breeders due to their seeming to
explode out of the ground after the first heavy rain in order to mate. They have short
legs and stocky bodies with vertical pupils. Characters: frontoparietal exostosed (with
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bony growths) (mainly in Pelobates and Scaphiopus); metatarsal spade supported by
well ossified hallux; smooth skin; parotid glands absent or not visible.

Ranidae (true frogs)
Members of this family are known as the true frogs and occur worldwide, have a limited
distribution in South America and Australia, but otherwise can be found throughout the
world.
Characters: long-legged; lack parotid glands and warts; webbed hind feet; dorsolateral
folds present for most species.

Helpful hints: How do you tell frogs and toads apart? A quick guide...but be careful.

FROGS







TOADS
1. Long hindlegs for jumping


Short hindlegs for hopping/ walking
2. Smooth typically moist skin


“Warty” typically dry skin
3. Many have dorsolateral folds


Large parotid glands
4. Feet tend to be webbed



Less webbing on feet
5. Live near water (most)



Live in drier habitats (most)



Station 5. Metamorphosis.
Amphibians differ from all other tetrapods in that they undergo a metamorphosis stage,
i. e., a more or less discrete transformation between larval and adult stages. Examine
the specimens at this station and note the obvious external changes that take place
during metamorphosis. How does metamorphosis differ between frogs and
salamanders? The amount of change may differ among species within the major
groups: for example, species that are highly aquatic as adults usually retain some
“larval” features that terrestrial species lose (e. g., lateral line system). A number of
amphibian species have direct development (young hatch out of egg as miniature adults
rather than as larvae). Many species of Plethodon (lungless salamanders) have direct
development. Notice that tadpoles and salamander larvae have the lateral line system,
like fishes, but adults typically lack this sensory system. How do the gills of salamander
larvae differ from those in
fishes?













Figure 5.1 Life cycle of a frog
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Station 6. Reproduction.
This station presents a small sample of the enormous diversity of reproductive
strategies used by amphibians. Only a few frog species have internal fertilization
(whereas this is typical for salamanders and caecilians). The tailed frog is one such
species and males use the “tail” for copulation. The Bufo woodhousii at this station
illustrates the vocal sac (used to attract mates) and forefoot nuptual pads (to help hold
onto a female) found in many male anurans. Leptodactylus occidentalis has spines on
the forefeet that may serve to grasp a female during copulation, but also may be used in
male-male combat. Dendrobatid frogs (poison arrow frogs) lay their eggs on land and
carry the newly hatched tadpoles on their backs to water. In some species females
place the eggs in the water-filled axils of bromeliad plants and then periodically return to
deposit unfertilized eggs on which the tadpoles feed. Males of many salamander
species, such as the Ensatina and Ambystoma at this station, have swollen cloacal
glands that may produce chemical secretions important in courtship.

Station 7. Locomotion.
Amphibians show adaptations for a variety of different locomotory modes. Specimens at
this station include arboreal, burrowing, highly aquatic, and semi-aquatic forms. Can
you guess which specimen uses each mode? Can you identify the characteristics
associated with each lifestyle?

from Gergus & Schuett 1997, Labs for Vertebrate Zoology: an Evolutionary Approach, Biological Sciences
Press
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Station 8. Warning coloration (aposomatism).
Many amphibians are brightly colored. In some cases coloration may function in species
recognition or courtship, but often its primary function is probably as a warning for
predators. All of the species at this station have noxious or poisonous secretions from
skin glands and are brightly colored on parts of their body (difficult to see in preserved
specimens because color fades). The California newt (Taricha torosa) and the fire-
bellied toad (Bombina orientalis) have bright colors on their bellies; when disturbed,
these species show stereotyped behaviors in which the back is arched to expose the
underside (= Unken reflex). The dendrobatid frogs in the picture have extremely toxic
alkaloids in their skin. These frogs are diurnal, which is unusual for frogs and is probably
related to their toxicity. These frogs are used by Choco Indians in South America to
poison blow-gun darts.

Station 9. Cave salamanders.
Several lineages of salamanders have independently evolved a cave-dwelling lifestyle.
Some features that have evolved over and over again in cave salamanders, such as
loss of skin pigmentation and reduction or loss of eyes, have obvious explanations.
Others, such as long limbs and reproduction as larvae (paedomorphosis), are more
perplexing. Try to think of adaptive explanations for the recurrent evolution of these
traits in cave salamanders. Check out Proteus anguinus (Proteidae) and Eurecea
speleaus (Plethodontidae).



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